1. Field of the Invention
This invention relates generally to a magneto-optic transducer for providing an optical indication of information magnetically recorded on a magnetic storage medium, and more particularly to novel geometry for such a transducer.
2. Description of the Prior Art
In the following description of the prior art, reference is made to FIGS. 1 through 4 of the accompanying drawings, in which:
FIG. 1 is a perspective view of a magneto-optic transducer known in the prior art;
FIGS. 2 and 3 are perspective and detailed elevational views, respectively, of a second magneto-optic transducer known in the prior art; and
FIG. 4 is an elevational view of a third magneto-optic transducer known in the prior art.
Magneto-optic transducers for providing an optical indication of information recorded on a magnetic storage medium are known in the prior art. A thin film of magneto-optic material is magnetized corresponding to magnetic fields produced by an adjacent magnetic storage medium. A beam of polarized light irradiates the magneto-optic material which causes amplitude variable, plus-or-minus rotation of the plane of polarization of the light, depending upon the strength and direction of magnetization of the material. The so-called Kerr effect occurs when light is reflected from a film of magnetized magneto-optic material, and the light has its plane of polarization rotated. When polarized light passes through magnetized magneto-optic material, the plane of polarization rotates by an amount proportional to the thickness traversed by the light and to the magnetic field strength. This is the Faraday effect. A light-responsive device is used to detect these optical variations.
FIG. 1 illustrates a Kerr effect type magneto-optic transducer 8 in accordance with the prior art. A prism 10 has a thin film 12 of magneto-optic material coated on the bottom face thereof. Light from a source 16 passes through a polarizer 18. As magnetic tape 14 moves under prism 10, thin film 12 of magneto-optic material is magnetized, and is "read out" by reflecting the polarized light off the magneto-optic material film and passing the reflected light through an analyzer 20 to a photodetector 22.
Two problems are known to occur with the prior art device shown in FIG. 1. Thin film of magneto-optic material is subject to wear because of moving contact with magnetic tape 14, and the resolution of the device is limited by the size of the light beam reflected off of the film of magneto-optic material. That is, the light beam generally has a cross-sectional dimension in the order of microinches. Accordingly, the smallest dimension of magnetization that can be resolved by such a beam is only in the order of microinches.
These problems of wear and resolution are not present in the prior art devices shown in FIGS. 2-4, and disclosed in U.S. Pat. No. 3,665,431 which issued on May 23, 1972 to J. K. Alstad et al. The Alstad et al. patent teaches a magneto-optic transducer with a thin film 24 (24') of magneto-optic material normal to magnetic tape 26 (26'). Since only an edge of the magneto-optic material film is immediately adjacent the magnetic tape, and that edge is transverse to tape movement, resolution is effectively equal to the film thickness. The thin film of magneto-optic material is supported between wear resistant glass prisms with reflective inner faces 28 and 30 (28' and 30') adjacent magnetic tape 26 (26'). The portion 32 (32') of the light beam reflected from film 24 (24') (Kerr effect) and the portion 34 (34') of the light beam refracted by the magneto-optic film (Faraday effect) is reflected from inner faces 28 and 30 (28' and 30'), respectively, and directed through analyzers to photodetectors.
Signal loss during playback of a magnetic storage medium is proportional to the separation between the thin film of magneto-optic and the storage medium, and the separation is itself proportional to the radius of curvature of the storage medium about the contacting edge of the thin film of magneto-optic material. The requirement of the Alstad et al. device for reflective faces 28 and 30 limits the angle of wrap about the edge of the thin film of magneto-optic material.